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Show 178 GEOLOGY OF THE HIGH PLATEAUS. The eastern front of the Tushar preserves that rugged mountainous aspect already described throughout two-thirds of its extent. The southern third is a wall of imposing grandeur, presenting to the eye the effect of a perpendicular escarpment, though really it is inclined at a slope of 60° or more. It is a magnificent object as seen from Circle Valley, rising nearly 2,000 feet above its base, and its base standing at the summit of a long slope which rises 2,000 feet above the valley bottom. This great cliff is a conglomerate composed of the ruins of older volcanic rocks. It is stratified, but not so conspicuously as most of the similar formations so abundant throughout the district. The finer material which incloses the rocky fragments is a light-gray pulverulent detritus, evidently resulting from the decomposition of feldspathic materials and highly aluminous. Some of the members of this series of heavy beds consist chiefly of this finer material, holding comparatively few fragments; in others the fragments are much more abundant, constituting the greater part of the mass. The fragments are usually somewhat rounded at the edges, but in most cases the amount of attrition is small, though seldom wholly unrecognizable. The mode of origin of this and similar conglomerates will be discussed in detail in a subsequent chapter. It is a sub-aerial formation throughout, and the mode of accumulation may be seen and studied hard by in all the valleys of the district. (See Heliotype II.) These beds are of ancient origin, having been formed prior to the great displacements which have given the Tushar its present structural features. The inclosed fragments are wholly variable in character. None of the rhyolitic, trachytic, and basaltic rocks of later age are seen among them, and the inference is irresistible that its formation was completed before these last-named masses were erupted. The source of these materials seems to have been the adjoining mass of the present Tushar table to the northward. To realize how this may have been we are obliged to go back in time to the later Eocene or early Miocene, when, in all probability, these great outbreaks occurred, and endeavor to reconstruct the country. At that time the centers or loci of eruption were doubtless in the very heart of the range, and stood considerably higher than the adjoining part of the country, just as they do now, though more recent movements on a grand |